@article {russell_rayleigh_2022,
	title = {Rayleigh Wave Attenuation and Amplification Measured at Ocean-Bottom Seismometer Arrays using Helmholtz Tomography},
	journal = {Journal of Geophysical Research: Solid Earth},
	volume = {n/a},
	number = {n/a},
	year = {2022},
	note = {_eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2022JB025174},
	month = {10/2022},
	pages = {e2022JB025174},
	abstract = {Shear attenuation provides insights into the physical and chemical state of the upper mantle. Yet, observations of attenuation are infrequent in the oceans, despite recent proliferation of arrays of ocean-bottom seismometers (OBS). Studies of attenuation in marine environments must overcome unique challenges associated with strong oceanographic noise at the seafloor and data loss during OBS recovery in addition to untangling the competing influences of elastic focusing, local site amplification, and anelastic attenuation on surface-wave amplitudes. We apply Helmholtz tomography to OBS data to simultaneously resolve array-averaged Rayleigh wave attenuation and maps of site amplification at periods of 20{\textendash}150 s. The approach explicitly accounts for elastic focusing and defocusing due to lateral velocity heterogeneity using wavefield curvature. We validate the approach using realistic wavefield simulations at the NoMelt Experiment and Juan de Fuca (JdF) plate, which represent endmember open-ocean and coastline-adjacent environments, respectively. Focusing corrections are successfully recovered at both OBS arrays, including at periods < 35 s at JdF where coastline effects result in strong multipathing. When applied to real data, our observations of Rayleigh wave attenuation at NoMelt and JdF revise previous estimates. At NoMelt, we observe a low attenuation lithospheric layer (Qμ>1500) overlying a highly attenuating asthenospheric layer (Qμ\~{}50{\textendash}70). At JdF, we find a broad peak in attenuation (Qμ\~{}50{\textendash}60) centered at a depth of 100{\textendash}130 km. We also report strong local site amplification at the JdF Ridge (>10\% at 31 s period), which can be used to refine models of crust and shallow mantle structure.},
	keywords = {amplification, attenuation, Helmholtz, ocean-bottom seismology, surface waves, tomography},
	issn = {2169-9356},
	doi = {10.1029/2022JB025174},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2022JB025174},
	author = {Russell, Joshua B. and Dalton, Colleen A.}
}
@article {ma_shear_2020,
	title = {Shear attenuation and anelastic mechanisms in the central Pacific upper mantle},
	journal = {Earth and Planetary Science Letters},
	volume = {536},
	year = {2020},
	month = {04/2020},
	pages = {116148},
	abstract = {We determine the mantle attenuation (1/Qμ) structure beneath 70 Myr seafloors in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: Qμ=1400 in the shallow layer, Qμ=110 in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, Qμ observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric Qμ measured at higher frequency (\~{}3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (VS) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (fe) and determine, at each depth, which values of fe predict Qμ and VS that can fit the NoMelt Qμ and VS values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in fe by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in fe reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at \~{}70-km depth.},
	keywords = {anelastic mechanism, oceanic lithosphere, Pacific Ocean, shear attenuation},
	issn = {0012-821X},
	doi = {10.1016/j.epsl.2020.116148},
	url = {https://www.sciencedirect.com/science/article/pii/S0012821X20300911},
	author = {Ma, Zhitu and Dalton, Colleen A. and Russell, Joshua B. and Gaherty, James B. and Hirth, Greg and Forsyth, Donald W.}
}
